Contiguous filter bank using shared resonators



Aug. 12, 1969 RUGGLEs ET AL 3,461,407

CONTLGUOUS FILTER BANK USING SHARED RESONATORS Filed March 5, 1966 FIG. 2.

PRIOR ART FIG. 4.

FIG. 3.

PRIOR ART INVENTORS No rmon A. Rugg les and lrvlng l. Koplun ATTORNEY PRlOR ART WITNESSES;

QHi ZUZW in/7% United States Patent 01 :"fice 5 Claims ABSTRACT OF THE DISCLOSURE Contiguous filter bank in which crystal resonators in contiguous channels are shared through isolating resistors and inductive coupling means, such as current transformers, each having two primary windings and a secondary winding constituting a mutually shared terminating output impedance. The ratio of the turns as between each primary and its associated secondary being such that the current through the terminating impedance is greater than that through the crystal resonators.

This invention relates to electrical signal filters and more particulalry to a bank of contiguous filters.

A bank of contiguous filters is defined as a group of bandpass filters having essentially bandpass characteristics, with the center frequencies of adjacent filters separatcd by an essentially constant frequency increment with each filter supplying its own individual load. Generally speaking, the separation between adjacent filters is not large compared with the bandwidth. Contiguous filter banks belong to a class of devices called spectrum analyzers" and have a wide variety of uses, such as for retrieving information signal data dispersed in a wide frequency signal power spectrum including noise and other frequencies of random phase and amplitude. Such devices are also useable in spectrum analyzers for determining all of the components of the frequencies in a spectrum which contributes to a time function.

Heretofore, in general, contiguous filter banks fall into two general classes. In one class, which is chronologically first, each individual filter and the elements thereof are used independently of the other except for possible loading effects on the source impedance. In the second class, res onators of adjacent filters are shared, that is, they are connected to function in the immediately adjacent channels, thus reducing the total number of resonant elements required to produce a bank of multipole filters.

The improved arrangement for sharing the resonators of adjacent filters in a contiguous bank in accordance with this invention is applicable to many uses for continuous filters and to many types of resonators. Since the apparatus of the present invention was developed in connection with a signal retrieval system for retrieving information signal data from the wide frequency power spectrum of Doppler radar, the invention will be described as applied to continuous filters used in a Doppler radar.

In a copending application, S.N. 563,830, filed July 8, 1966, in the names of Roger J. McGee and Ralph Metz, which application is owned by the assignee of this invention, there is disclosed and claimed an improved signal translation system in which the receiver includes a contiguous filter bank for retrieving the signal data information from a Doppler radar spectrum, in which the adjacent filters use quartz crystal resonators time shared between adjacent filters. The contiguous filter bank used in that signal translation system is the subject of the present invention.

3,461,407 Patented Aug. 12, 1969 -It is readily apparent that the primary advantage of using shared resonators in a contiguous filter bank is in the reduction in the number of resonators required. For example, a bank of N filters, each using two resonant elements per filter, requires (2 N) resonators using conventional filters and only (N-l-l) resonators with the sharing technique. A real problem presented by the resonator-sharing arrangement is that of having all of the filters connected to a common input bus so that each and every filter is available to pick up any signal component within the band ofthe filter while at the same time pro- 1 viding a minimum of coupling between the filters in order to avoid a false or inaccurate signal identification.

-Accordingly, the primary object of the present invention is to provide a bank of contiguous filters in which resonators of adjacent filters are shared with a minimum of signal dissipation and a minimum of cross talk between channels.

Other and further objects of the invention will best be understood from the following description when taken in connection with the accompanying drawing, in which:

FIGURES 1 and 2 depict, in general, two previously known arrangements for mechanizing the shared resonator technique in contiguous filter banks;

FIGS. 3 and 4 show, in general, two prior filter arrangements, FIG. 3 showing a voltage transformer coupling and FIG. 4 showing a current transformer coupling; and

FIG. 5 is a circuit diagram of the present invention.

The present invention can best be pointedly presented by pointing out the differences between FIGS. 1 to 4, inclusive, on one hand and FIG. 5 on the other hand. The nature of circuit arrangement of FIG. 1 is such that it is desirable to utilize a common input transformer 10 with a center tap secondary 11. The two terminals of the secondary of transformer 10 are connected to two bus bars 12 and 13, respectively, which are connected to the crystals 16. Further study of this circuit will indicate that it is necessary that the current summing points 15 at the input to the summing amplifiers 14 must be held at virtual ground in order to minimize cross talk between the channels. It will also be noted that with this arrangement, each of the crystals 16 is connected between one of the bus bars and a circuit which includes two resistors 17 connected in parallel and, therefore, only half of the current flowing through a given resonator fiows into a given summing point. This is a substantial disadvantage where there are a large number of filters which must be connected in parallel, such as in Doppler radar.

The fact that total crystal current is twice the contribution of the crystal to the summing point current presents obvious difliculties because (a) the amount of power required to be transmitted by by the filter can increase above normal noise level by the ratio of total contiguous bank bandwidth to the filter bandwidth if a strong target return which captures the AGC of the amplifier driving the bank is present;

(b) the usual load for the filter is a detector of some sort and normally this load has an input impedance which is a function of signal level and temperature, requiring some sort of buffering for the filter to keep the termination impedance constant, and a minimum input level below which satisfactory detection cannot be obtained. The minimum acceptable output signal for the filter is thus fixed by the threshold requirements of the detector, while the maximum possible input signal to the filter is fixed by the peak-to-peak swing capabilities of the amplifier driving the bank; and

(c) where quartz crystals are used as the resonators, they inherently have a limited power dissipation capacity. Thus, when a large number of filters are driven by a single amplifier the wide range in the input signal may overload the quartz crystals. It is evident that in the prior art the resonator current in all instances must be twice that at the summing point 15 coming from the resonator. In practice, therefore, it is necessary to provide a current gain through the summing amplifier and in the case of FIGURE 2, a stable alternate phase reversal. It does not take very much discussion to show that the system of FIGS. 1 and 2 would have a disadvantage in comparison with any kind of system which would eliminate the two to one ratio of current through the resonator crystals to that delivered to the summing points at the inputs to the amplifiers.

In the arrangement shown in FIG. 5, in accordance with the present invention, it is immediately apparent that the current supplied to the virtual ground of the amplifiers 20 can be greater than that flowing through the crystals 23. The amplifiers have low input impedance so that their input terminals 21 are substantially at ground potential. The signal input terminal to the filter is indicated at 22. This point is coupled to the respective amplifiers through the quartz filter crystals 23, isolating resistors R1 and R2 and current transformers 24. The transformers are identical and in the illustrated embodiment the toroidal transformers each have two primary windings 24a and 24b and a secondary winding 240. One side of the secondary winding 240 of each transformer is connected to the respective input terminals 21 of the amplifiers 20. The other terminal of the secondary winding 240 is connected to a common ground bus bar indicated at 26.

It is to be noted that each of the primary windings 24a of the transformer for each channel is connected in a series circuit between the input terminal 22 and the ground 26 which includes one of the crystals 23, an isolating resistor, one of the primary windings of the transformer for one adjacent channel and the secondary of the transformer for the first mentioned channel, The primary windings on the transformers are so poled as to provide the required phase reversal, just as in the current transformer of FIGURE 4. In this way, it will be noted that each resonator total current is shared inductively in adjacent channels. At the same time, the isolating resistors provide an essentially constant terminating impedance for the resonators. As compared to the arrangements in FIGS. 1 and 2, it is to be noted that in FIG. 5 there is only one isolating resistor per filter channel while the turns ratio of the transformer provides the very desirable current gain between resonator current and filter output current.

The term filter as used herein describes a circuit which includes a crystal resonator, a resistor, the primary winding of an adjacent transformer and the terminating impedance. In the illustrated embodiment the terminating impedances are the inputs of the transformers.

What is claimed is: r 1. A contiguous filter bank comprising a plurality of bandpass filters with the center frequencies of adjacent filters being separated by an essentially constant frequency increment, each of said filters being connected to a common input bus, each of said filters having a resonator element and an isolating resistor which are shared with adjacent filters through inductively coupled means, said resistors serving to effectively isolate said filters, said inductive coupling means constituting mutually shared terminating impedances for the adjacent filters and provides higher current through the terminating impedances than through the respective crystals and their isolating re-- sistors.

2. The combination as set forth in claim 1 in which said magnetomotive means are respective transformers, each having two primaries and a single secondary, each of said secondaries having one side thereof connected to a common ground and the other respective sides thereof connected to respective filter amplifiers, a primary winding of each of the respective transformers of adjacent channels being connected in series with a respective resonator and isolating resistor, whereby the total current through each resonator is shared inductively with adjacent filters, and the current flowing in the terminating load of the filters is greater than that flowing through said resonators by a factor determined by the turns ratio of said transformers.

3. The combination as set forth in claim 2 in which said transformers are current transformers and have a turns ratio as regards one primary with respect to the secondary greater than unity.

4. The combination as set forth in claim 2 in which the primary windings of adjacent transformers are poled in opposing phase relation to substantially match the substantial 180 phase differential between adjacent resonators whereby the signals through the adjacent filters are added in phase in the secondary of said current transformers.

5. The combination as set forth in claim 2 in which said transformers are toroidal and each has two primary windings with a turns ratio greater than unity as regards each primary with respect to the secondary,

References Cited UNITED STATES PATENTS 1,802,738 4/1931 Roberts 333-6 2,907,838 10/1959 Ross 333 X HERMAN K. SAALBACH, Primary Examiner P. L. GENSLER, Assistant Examiner US. Cl. X.R. 

